This invention relates generally to processes for removing sulfur oxides from gas streams and more particularly to the regeneration of scrubbing solutions used in a scrubber to remove sulfur oxides from gas streams.
It is common practice to remove sulfur oxides, especially sulfur dioxide, from a gas stream by passing the gas stream through a scrubber which contacts the gas stream with an aqueous scrubbing solution containing sodium chemicals that react with the sulfur oxides to remove them from the gas stream. Typically, the sodium chemicals in the scrubbing solution are predominately sodium sulfite, sodium bisulfite and sodium sulfate. The sulfur dioxide in the gas stream will react with sodium sulfite in the scrubbing solution to convert some of the sodium sulfite to sodium bisulfite while removing the sulfur dioxide from the gas stream. If the scrubbing solution contains any sodium hydroxide or sodium carbonate as is sometimes the case, the sulfur dioxide in the gas stream will react with either of these sodium chemicals to form sodium sulfite while removing the sulfur dioxide from the gas stream. The scrubbing solution is separated from the gas stream so that a liquid free gas stream is exhausted from the scrubber. The separated scrubbing solution is collected and recirculated back through the scrubber for further contact with the gas stream to conserve the sodium values in the scrubbing solution.
As the sulfur dioxide reacts with the sodium sulfite in the scrubbing solution, the sodium sulfite concentration will be reduced and thus will reduce the ability of the scrubbing solution to remove sulfur dioxide. One way to maintain the sodium sulfite concentration in the scrubbing solution and thus maintain its sulfur dioxide removal capability is to bleed off and dispose of part of the scrubbing solution while replacing the disposed solution with an aqueous makeup solution containing a sodium chemical that reacts with sodium bisulfite in the scrubbing solution to convert it back to sodium sulfite and/or with the sulfur dioxide to form sodium sulfite. This technique for maintaining the sodium sulfite concentration in the scrubbing solution consumes large quantities of sodium chemicals and also generates a liquid waste for disposal. The costs associated with this technique for the acquisition of the sodium chemicals and for the disposal of the liquid waste are relatively high.
Another way to maintain the sodium sulfite concentration in the scrubbing solution is to bleed off and regenerate part of the scrubbing solution using a calcium compound to convert the sodium bisulfite back to sodium sulfite. U.S. Pat. Nos. 3,911,084 and 3,989,797 disclose this technique. In both of these patents, calcium hydroxide (slaked lime) is reacted with the scrubbing solution to convert the sodium bisulfite back to sodium sulfite and generate insoluble calcium sulfite. The calcium sulfite is then separated from the solution and the regenerate solution returned to the primary scrubbing solution for recirculation in the scrubber. While both of these patents mention that calcium oxide (lime) might be used to regenerate the scrubbing solution, attempts to use calcium oxide have not met with practical success because no way was known to react the calcium oxide with the spent scrubbing solution fast enough to produce a commercially feasible process. As a result, the regeneration of spent scrubbing solutions have in the past been limited to the use of calcium hydroxide (slaked lime). This requirement has increased the cost of the regeneration process since calcium oxide must be hydrated in a slaker to form the slaked lime before being used in the process. Further, the use of slaked lime in the process required careful handling of the calcium sulfite formed in the regenerated solution so that it could be separated from the solution.